This invention relates to in situ measurements of earth formations traversed by a well borehole. More particularly the invention relates to the measurement of thermal neutron decay time or neutron lifetime of earth formations in the vicinity of a well borehole and of the borehole itself.
In recent years, an improved well logging system for measuring simultaneously the thermal neutron decay components of the earth formations in the vicinity of a well borehole and the borehole component itself had been introduced. U.S. Pat. No. 4,409,481 which is assigned to the assignee of the present invention and which is incorporated by reference herein in its entirety describes the most successful commercial system for making such simultaneous measurements. Briefly in the invention described in the U.S. Pat. No. 4,409,481, a well logging tool is moved through a borehole which includes a pulsed source of fast neutrons and two radiation detectors. The pulsed neutron source generates a pulse of neutrons of approximately 14 Mev energy for a duration of between 10 and 150 microseconds at a pulse rate of approximately 1,200 pulses per second. The neutrons are introduced into the media comprising the well borehole and the surrounding earth formations and result in a thermal neutron population being generated from the slowing down of the fast neutrons in the earth formation media and in the borehole media. After a very short pause to allow moderation of the fast neutrons following the neutron pulse, the detectors are gated on and capture gamma radiation resulting from the capture of thermal neutrons in the borehole and the earth formations in the vicinity of the borehole are measured essentially continuously until the next neutron burst is about to begin. Multiple time gates which occur during this essentially continuous interval are used for this purpose. In the U.S. Pat. No. 4,409,481, the use of six such time gates is described. The gates, being essentially contiguous in time and of variable length, have the shorter gates closer to the neutron pulse and the longer duration gates being further removed in time from the neutron pulse.
The contiguous nature of the gates tends to reduce statistical errors, but neither the number of or continuous nature of the gates are essential to this invention, so long as the number and position of gates are adequate to provide data to solve for the desired components. The multiple time gate measurements of the counting rates of the gamma rays measured in each gate are supplied to a thermal neutron lifetime computer which computes formation and borehole neutron lifetime components by means of an iterative least squares fitting technique of the count rate data taken during four or more of the time gates following each neutron burst. The thermal neutron lifetime computer is enabled to calculate both the borehole thermal neutron lifetime component and the earth formation thermal neutron lifetime component simultaneously, and also simultaneously can compute the magnitude of the initial borehole and initial formation neutron components of thermal neutron population.
Approximately once per second and for approximately 5% of the one second operating cycle, the neutron source pulsing sequence is turned off completely and the detectors are used to establish any relatively long lived background counting rate due to source neutron induced gamma ray activity within the gamma ray detector, the formation, the borehole, the logging sonde or any natural gamma radiation in the vicinity of the borehole. This background gamma radiation information is then properly normalized and subtracted from the count rates made in each of the time gates following the neutron bursts. The percentage of the one second cycle used for background can be varied but the approximate 5% amount mentioned has been found to be suitable for this purpose.
A problem which has arisen due to the statistical nature of the measurement of the gamma rays generated by captured thermal neutrons has been that certain mathematical filtering techniques must be applied both to the raw measurement data prior to the data processing and then to the computed thermal neutron decay time or lifetime parameters for both the borehole and the formation which are produced by the logging system described previously. Moreover, the detectors in the system described for thermal multigate decay time logging are spaced as close as conveniently possible to the neutron source in the downhole logging instrument but, because of the relatively small diameter of the logging instrument, the near detector is spaced a distance of approximately 12 inches from the neutron source and the far detector is approximately 12 inches further from the neutron source than the near detector. A rough rule of thumb of vertical resolution of a logging instrument is that resolution is proportional to the distance between the source and the detector simply because of the physics of source propagation (of most types) and the received stimulus observed by the detector. This is not strictly true in the instance of pulsed neutron logging tools such as the thermal multigate decay log in that the vertical resolution of the long spaced detector is not quite twice that of the short spaced detector, but both have resolutions in the range of 18-30 inches.
It is thus seen that, because of the statistical count data filtering and the spacing of source to detectors in the thermal multigate decay time logging system, vertical resolution on the order of 4 to 5 feet is to be expected with the conventionally processed data from this type of logging device. Because of the commercial realities of life, however, it is becoming more important to be able to evaluate thin bed stringers inside of shaley sand formations for potential hydrocarbon production. Higher vertical resolution than the 4 to 5 feet resolution provided by conventionally processed data from the logging instrument in a thermal multigate decay system is quite desirable. In the present invention, such higher resolution is provided by unique processing techniques which are based upon the physics of the measurement, but which employ mathematical processing techniques in their implementation to improve the vertical resolution offered by the measuring instrument and processing software.